This invention relates to thermosetting composite emulsion polymers and more particularly to an in-situ formed reactive emulsion polymer produced in the presence of a glycoluril crosslinker.
Glycoluril compositions are known for use in solvent based coating compositions such as disclosed in U.S. Pat. No. 4,064,191. Powder paints based on glycoluril are disclosed in U.S. Pat. Nos. 4,254,235, 4,255,558, and 4,118,437. Water dispersed emulsion polymers based on glycoluril mixtures are disclosed in U.S. Pat. No. 4,442,257 and commonly assigned U.S. Ser. No. 413,728 filed Sept. 1, 1982, (U.S. Pat. No. 4,444,941) and the disclosures thereof are incorporated herein by reference.
Aqueous latex coating compositions based on a thermosetting combination of glycoluril adapted to cross-link with a reactive polymer containing reactive carboxyl, hydroxyl, or acrylamide groups provide a low temperature cure surface coating. Low temperature water based coatings are highly desirable since less energy is required for curing and organic solvent emission pollution is avoided. The reactive emulsion polymer will cure with the coreactive glycoluril at surprisingly low temperatures of about 40.degree. C. to 120.degree. C. for 0.1 to 20 minutes, and advantageously less than 3 minutes. This quick and low temperature cure cannot be achieved with similar polymers now with conventional cross-linking compounds such as melamine.
It now has been found that coatings based on the combination of glycoluril and reactive emulsion polymer can be substantially improved by copolymerizing the monomers for synthesizing the reactive emulsion polymers in the presence of the glycoluril dispersed in water to produce an in-situ formed composite of reactive emulsion polymer and glycoluril. By synthesizing the reactive emulsion polymer in the presence of glycoluril, a better and more uniform dispersion of emulsion polymer results whereby a better incorporation of crosslinker into the emulsion polymer particles is formed in comparison to simple blending. Synthesizing the latex in an aqueous dispersion of glycoluril crosslinker/water solution, is believed to provided free-radical grafting on the crosslinker and some reaction of the crosslinker with hydroxyl functional monomer to give better incorporation of crosslinker into the particles thus preventing its diffusion into porous substrates.
The process of this invention provides coating compositions which provide uniform cured coating films, consistently clear films, and improved uniform gloss. The films applied over porous substrates avoids diffusion of one component such as glycoluril in preference to the matrix emulsion polymer and generally provides improved film integrity properties. Better interdispersion of the latex and glycoluril provides improved holdout and can enable lesser amounts of glycoluril to obtain comparable film physical properties. These and other advantages will become more apparent by referring to the detailed description and the illustrative examples.